In recent years, the increasing market demand on wireless network products has pushed the development of wireless local area network (wireless LAN) to a much higher level. Particularly, the IEEE 802.11 protocol established in the year 1997 provides many new functions in the wireless LAN communications and a solution for communicating between wireless products of different brands, which opens up a new milestone for the development of wireless LAN applications. This protocol sets the direction of wireless LAN development to a single chip solution running on a kernel device. This largely reduces the cost of wireless technology, thus making the wireless LAN technology being applicable to various kinds of wireless products.
Moreover, in order to satisfy the compactness requirement of wireless network products in the market, the manufacturers thereof must design the circuit layout of the wireless network products in a strictly limited space. Consequently, when a wireless network product is working under a high power condition, harmonic waves of high frequencies are generated due to the nonlinear properties of the active components, thereby causing electromagnetic interference (EMI). In order to solve the problem described above, many developed countries have established statutory restrictions to limit the import or usage of such wireless network products. For this reason, the manufacturers of such wireless network products must include a filter therein to comply with the statutory requirements and to suppress the generated noise of harmonic waves of high frequencies, when designing their wireless network products, especially when the power is amplified. The harmonic waves of high frequencies of the manufactured wireless network products can thus meet the statutory requirements of all countries.
Let's take a printed circuit of a wireless network product working under a frequency of 2.4 GHz as an example, as shown in FIG. 1. In order to reduce the manufacturing cost and space occupied by the filter, a low pass filter having a size of 133×300 mil2 disposed after the power amplifier is normally fabricated on the printed circuit board. The filter is formed directly on the extra space of the printed circuit board, which is left out from the wiring scheme of the microstrip lines. The filter comprises a microstrip input end 10. One end of the microstrip input end 10 is perpendicularly connected to one end of a first microstrip line Z1. The filter further comprises a microstrip output end 15. One end of the microstrip output end 15 is perpendicularly connected to one end of a third microstrip line Z3. The first microstrip line Z1 and the third microstrip line Z3 are mutually spaced and symmetrically arranged wherein a second microstrip line Z2 is connected therebetween. The second microstrip line Z2 is in a meandering shape, which is being disposed between the first and the third microstrip lines Z1, Z3. In addition, the two ends of the second microstrip line Z2 are perpendicularly connected to the microstrip input line 10 and the microstrip output line 15. The necessary low pass filter is thus formed to effectively suppress the noise of harmonic waves of high frequencies, which is generated from the power amplifier due to nonlinear distortions.
One can use the low pass filter shown in FIG. 1 to filter the wave signals generated from a wireless network product at a working frequency of 2.4 GHz. The resultant noise is measured and illustrated in FIG. 2. As shown, the insertion loss at 2.4 GHz is approximately 0.0751 dB, while the suppression is approximately 27.73 dB at a double frequency of 4.8 GHz. However, the first microstrip line Z1 and the third microstrip line Z3 of such low pass filter occupy quite a large area on the printed circuit board. This hinders the printed circuit board of wireless network products from further shrinking down to a smaller size. Therefore, the problem of how to shrink down the size of wireless network products without increasing the manufacturing cost and yet satisfying the related statutory requirements is a very important issue to solve.